Additive color digital image display devices are well known and are based upon a variety of technologies such as cathode ray tubes, liquid crystal modulators, and solid-state light emitters such as Organic Light Emitting Diodes (OLEDs). In a common OLED color display device a pixel includes red, green, and blue colored OLEDs. These light emitting color primaries define a color gamut, and by additively combining the illumination from each of these three OLEDs, i.e. with the integrative capabilities of the human visual system, a wide variety of colors can be achieved. OLEDs may be used to generate color directly using organic materials that are doped to emit energy in desired portions of the electromagnetic spectrum, or alternatively, broadband emitting (apparently white) OLEDs may be attenuated with color filters to achieve red, green and blue.
It is possible to employ a white, or nearly white OLED along with the red, green, and blue OLEDs to improve power efficiency and/or luminance stability over time. Other possibilities for improving power efficiency and/or luminance stability over time include the use of one or more additional non-white OLEDs. However, images and other data destined for display on a color display device are typically stored and/or transmitted in three channels, that is, having three signals corresponding to a standard (e.g. sRGB) or specific (e.g. measured CRT phosphors) set of primaries. It is also important to recognize that this data is typically sampled to assume a particular spatial arrangement of light emitting elements. In an OLED display device these light emitting elements are typically arranged side by side on a plane. Therefore if incoming image data is sampled for display on a three color display device, the data will also have to be resampled for display on a display having four OLEDs per pixel rather than the three OLEDs used in a three channel display device.
In the field of CMYK printing, conversions known as undercolor removal or gray component replacement are made from RGB to CMYK, or more specifically from CMY to CMYK. At their most basic, these conversions subtract some fraction of the CMY values and add that amount to the K value. These methods are complicated by image structure limitations because they typically involve non-continuous tone systems, but because the white of a subtractive CMYK image is determined by the substrate on which it is printed, these methods remain relatively simple with respect to color processing. Attempting to apply analogous algorithms in continuous tone additive color systems would cause color errors if the additional primary is different in color from the display system white point. Additionally, the colors used in these systems can typically be overlaid on top of one another and so there is also no need to spatially resample the data when displaying four colors.
In the field of sequential-field color projection systems, it known to use a white primary in combination with red, green, and blue primaries. White is projected to augment the brightness provided by the red, green, and blue primaries, inherently reducing the color saturation of some, if not all, of the colors being projected. A method proposed by Morgan et al. in U.S. Pat. No. 6,453,067 issued Sep. 17, 2002, teaches an approach to calculating the intensity of the white primary dependent on the minimum of the red, green, and blue intensities, and subsequently calculating modified red, green, and blue intensities via scaling. The scaling is ostensibly to try to correct the color errors resulting from the brightness addition provided by the white, but simple correction by scaling will never restore, for all colors, all of the color saturation lost in the addition of white. The lack of a subtraction step in this method ensures color errors in at least some colors. Additionally, Morgan's disclosure describes a problem that arises if the white primary is different in color from the desired white point of a display device without adequately solving it. The method simply accepts an average effective white point, which effectively limits the choice of white primary color to a narrow range around the white point of the device. Since the red, green, blue, and white elements are projected to spatially overlap one another, there is no need to spatially resample the data for display on the four color device.
A similar approach is described by Lee et al. (SID 2003 reference) to drive a color liquid crystal display having red, green, blue, and white pixels. Lee et al. calculate the white signal as the minimum of the red, green, and blue signals, then scale the red, green, and blue signals to correct some, but not all, color errors, with the goal of luminance enhancement paramount. The method of Lee et al. suffers from the same color inaccuracy as that of Morgan and no reference is made to spatially resampling of the incoming three color data to the array of red, green, blue and white elements.
In the field of ferroelectric liquid crystal displays, another method is presented by Tanioka in U.S. Pat. No. 5,929,843, issued Jul. 27, 1999. Tanioka's method follows an algorithm analogous to the familiar CMYK approach, assigning the minimum of the R,G, and B signals to the W signal and subtracting the same from each of the R, G, and B signals. To avoid spatial artifacts, the method teaches a variable scale factor applied to the minimum signal that results in smoother colors at low luminance levels. Because of its similarity to the CMYK algorithm, it suffers from the same problem cited above, namely that a white pixel having a color different from that of the display white point will cause color errors. Similarly to Morgan et al. (U.S. Pat. No. 6,453,067, referenced above), the color elements are typically projected to spatially overlap one another and so there is no need for spatial resampling of the data.
It should be noted, that the physics of light generation and modulation of OLED display devices differ significantly from the physics of devices used in printing, display devices typically used in field sequential color projection, and liquid crystal displays. These differences impose different constraints upon the method for transforming three color input signals. Among these differences is the ability of the OLED display device to turn off the illumination source on an OLED by OLED basis. This differs from devices typically used in field sequential display devices and liquid crystal displays since these devices typically modulate the light that is emitted from a large area light source that is maintained at a constant level. Further, it is well known in the field of OLED display devices that high drive current densities result in shorter OLED lifetimes. This same effect is not characteristic of devices applied in the before-mentioned fields.
While stacked OLED display devices have been discussed in the prior art, providing full color data at each visible spatial location, OLED display devices are commonly constructed from multiple colors of OLEDs that are arranged on a single plane. When displays provide color light emitting elements that have different spatial location, it is known to sample the data for the spatial arrangement. For example, U.S. Pat. No. 5,341,153 issued Aug. 23, 1994 to Benzschawel et al., discusses a method for displaying a high resolution color image on a lower resolution liquid crystal display in which the light emitting elements of different colors have different spatial locations. Using this method, the spatial location and the area of the original image that is sampled to produce a signal for each light emitting element is considered when sampling the data to a format that provides sub-pixel rendering. While this patent does mention providing sampling of the data for a display device having four different color light emitting elements, it does not provide a method for converting from a traditional three color image signal to an image signal that is appropriate for display on a display device having four different color light emitting elements. Additionally, Benzschawel et al. assumes that the input data originates from an image file that is higher in resolution than the display and contains information for all color light emitting elements at every pixel location.
The prior art also includes methods for resampling image data from one intended spatial arrangement of light emitting elements to a second spatial arrangement of light emitting elements. US Patent Application No. 2003/0034992A1, by Brown Elliott et al., published Feb. 20, 2003, discusses a method of resampling data that was intended for presentation on a display device having one spatial arrangement of light emitting elements having three colors to a display device having a different spatial arrangement of three color light emitting elements. Specifically, this patent application discusses resampling three color data that was intended for presentation on a display device with a traditional arrangement of light emitting elements to three color data that is intended for presentation on a display device with an alternate arrangement of light emitting elements. However, this application does not discuss the conversion of data for presentation on a four or more color device.
There is a need, therefore, for an improved method for transforming three color input signals, bearing images or other data, to four or more output signals.